1. Field of the Invention
The present invention relates to organically modified fine particles having hydrocarbon strongly bound to the surface of fine particles, particularly, organically modified metal oxide fine particles, a process for producing the same, and further a recovery or collection method of fine particles such as nanoparticles, and applied techniques thereof.
2. Description of the Related Art
Fine particles, particularly, particles of nanometer size (nanoparticles) are expected to realize a technology satisfying requests of a higher precision, a smaller size and a lighter weight than in the current condition for all materials and products because of a variety of unique excellent properties, characteristics and functions thereof. Therefore, nanoparticles are attracting attentions as a material enabling the higher function, higher performance, higher density, and higher preciseness of industrial materials, pharmaceutical and cosmetic materials and the like such as ceramic nano-structure modified material, optical functional coating material, electromagnetic shielding material, secondary battery material, fluorescent material, electronic part material, magnetic recording material, and abrasive material, and also as a 21st century material. There is a lot of attention from the industrial world being given thereto, because a series of discoveries such as onset of extra-high functionalities or new physical properties by the quantum size effect of the nanoparticles, and syntheses of new materials have been made in the recent basic researches for the nanoparticles. However, practical application of the nanoparticles requires addition of unique functions to the respective fine particles, and to that end, establishment of a technique for modifying the surface of particles is desired for enabling the addition of the functions. Organic modification is convenient for adding stably usable and applicable functions to fine particles, particularly nanoparticles, and a modification through strong bond is particularly demanded.
Many reports for organic/inorganic composite materials and syntheses thereof have been disclosed up to now, and researches for organic modification of inorganic particles have been also made, each of which intended to carry out a reaction in an organic solvent for the organic modification. A technique for reacting an organic material with fine particles in a reaction field or water while synthesizing the particles in water or an aqueous solution is not known. For the surface modification of particles, many techniques for modifying the surface of inorganic particles in an organic solvent are known. However, particles of nano-size are easy to coagulate, and a pretreatment such as use of a surface active agent is particularly needed for dispersing the particles synthesized in water to an organic solvent. As mentioned above, no report has been made for the technique for modifying the surface of particles while synthesizing the particles in water.
As a method for in-situ surface modification, reversed micelle method, hot soap method, and the like have been reported. In the reversed micelle method, water is suspended to an oil phase by use of a surface active agent to generate a reversed micelle, and a reactive substrate is added thereto to reactively crystallize it. The metal oxide particles generated in the suspended water phase are stabilized by the surface active agent to stably disperse nanoparticles, in which the surface active agent is in a state adsorbed by the particle surface without a linkage by reaction. In the hot soap method, the above method is performed at high temperature by use of only the surface active agent without oil phase. This method is a technique using the effect that an aqueous solution of metal salt to be reacted is rapidly supplied with stirring and reactively crystallized, while the circumferential surface active agent is adsorbed thereto. The cases reported up to now for the organic modification involve adsorption of alkanethiol, but not reactive modification.
There are frequently reported that high-temperature, high-pressure water forms a homogeneous phase also with an organic material, and water functions as an acid or basic catalyst in a high-temperature, high-pressure field to progress an organic synthetic reaction even under no catalyst. However, no method for a reaction between an inorganic material and an organic material is reported.
It is known that highly crystalline particles of nano-size can be synthesized by adapting supercritical water as a reaction field for hydrothermal synthesis. However, there is no report for modification of the surface of the produced nanoparticles in this reaction field, or synthesis of organically modified particles by a reaction with an organic material.
A technique for performing an in-situ surface modification simultaneously with CVD in a supercritical fluid is also known, in which alkanethiol or alcohol is made coexist in a reaction field for synthesizing metal nanoparticles by CVD in the supercritical fluid, taking reference to the above-mentioned hot soap method or the like. It is reported that the growth of particles can be inhibited to generate particles of nanometer according to this technique. For the CVD technique, it is reported that the reduction reaction and surface modification by alkanethiol simultaneously occur, and the resulting product is metal Cu having an alkanethiol-coordinated structure. It is also reported to produce nanoparticles by performing the synthesis in the presence of alkanethiol similarly to the above by use of a reducing agent in supercritical water, thereby coordinating the thiol group with metal nanoparticles to inhibit the growth of particles. In case of the alcohol, it is shown in a part of the result that not only orientation but also linkage was caused to perform in-situ surface modification in the reaction filed. However, this is caused not by the reactive crystallization in supercritical “water”, but by only a technique belonging to reactive in-situ surface reforming method in an organic solvent.
A surface treatment of glass or silica gel in water is well known, but this method is based on CNBr activation or epoxy activation. Since each of the CNBr method and the epoxy activation method is carried out in an alkaline solution, particles of about nanometer (nm) are entirely dissolved. Therefore, these known reactions cannot be used for the surface modification of oxide nanoparticles in water.
Conventional organic modification methods will be collectively described.
1) Synthesis of Organic-Inorganic Composite Material
Silane coupling is given as a general method for modifying a metal oxide surface. (Polymer Frontier 21 Series 15 “Inorganic/Polymer Nano Interface Control”, edited by Society of Polymer Science, pp. 3-23, NTS, 2003). There are researches for syntheses of organic/inorganic composite materials [“Formation of Ordered Monolayer of Anionic Silica Particles on a Cationic Molecular Layer”, T. Yonezawa, S. Onoue, and T. Kunitake, Chem. Lett., No. 7, 689-690 (1998); Molecular Imprinting of Azobenzene Carboxylic Acid on a TiO2 Ultrathin Film by the Surface Sol-Gel Process”, S.-W. Lee, I. Ichinose, T. Kunitake, Langmuir, Vol. 14, 2857-2863 (1998); “Alternate Molecular Layers of Metal Oxides and Hydroxyl Polymers Prepared by the Surface Sol-Gel Process”, I. Ichinose, T. Kawakami, T. Kunitake, Adv. Mater., Vol. 10, 535-539 (1998); and “Molecular Imprinting of Protected Amino Acids in Ultrathin Multilayers of TiO2 Gel”, S. W. Lee, I. Ichinose, T. Kunitake, Chem. Lett., No. 12, 1993-1994 (1998)]. Surface modifications of oxides in water are also known. Techniques for surface-modifying glass or silica gel in water include CNBr activation or epoxy activation, in which CNBr or epoxy is reacted with OH on the surface to give the functional group of the CN or epoxy, and an intended functional group is introduced through it. However, such a reaction requires setting of pH and addition of a catalyst, and involves generation of an acid as a product. Since these activations are carried out in an alkaline solution, all particles of about nm are dissolved, and it is therefore impossible to perform the surface modification of oxide nanoparticles in water by use of these known reactions (Rolf Axen, Jerker Porath, Sverker Embvack, “Chemica Coupling of Peptides and Proteins to Polysaccharides by Means of Cyanogen Halides”, Nature, Vol. 214, 1967, pp. 1302-1304). In every method described above, the organic modification depends on a reaction in an organic solvent. Particles of nano-size are easy to coagulate because of high surface energy. A solution method such as sol-gel process or hydrothermal process is effective, as shown in FIG. 1, in synthesizing particles of 10 nm or less. However, the particles synthesized in a solvent are firmly coagulated, when taken out and dried, and it is extremely difficult to redisperse them in an organic solvent. The solvent must be changed to the organic solvent stepwise. Particularly, the nanoparticles synthesized in water frequently have hydrophilic groups, and a pretreatment such as use of a surface active agent is needed for dispersing them to an organic solvent. Accordingly, the technique for surface-modifying nanoparticles in situ while synthesizing them is important for synthesis of nano-particles of 50 nm or less.
2) Technique for Performing In-Situ Surface Modification
Reversed Micelle Method
Water is suspended in an oil phase by use of a surface active agent to form a reversed micelle, and a reactive substrate is added thereto to reactively crystallize it. For example, CdS nanoparticles and NaNO3 can be generated by mixing an aqueous solution of Cd(NO3)2 with micelle of Na2S. The CdS nanoparticles can be stabilized by supplying a stabilizing agent such as alkanethiol. The surface active agent is in a state adsorbed to the surface without a linkage by reaction (A) New Technology for Production, Evaluation, Application and Equipment of Nanoparticles”, pp. 16-19, 2002, published by CMC). Recently, a method using supercritical carbon dioxide as solvent has been also reported (Ye, X. R., Lin, Y. Wang, C., Wai, C. M., Adv. Materials, 2003, 15, 316; and Ye, X. R., Lin, Y. Wang, C., Wai, C. M., Chem. Comm., 2003, 642).
Hot Soap Method
The above-mentioned method is carried out at high temperature by use of only a surface active agent without oil phase. An aqueous solution of metal salt to be reacted is rapidly supplied with stirring and reactively crystallized, while the circumferential surface active agent is adsorbed thereto (A) “New Technology for Production, Evaluation, Application and Equipment of Nanoparticles”, pp. 19-21, 2002, published by CMC). Most of the cases reported up to now are based on adsorption of alkanethiol, and no reactive modification has been practiced.
In-Situ Surface Modification in Supercritical Fluid by Reactive Crystallization
A method for performing an organic modification simultaneously with thermal decomposition CVD (chemical vapor deposition) in a supercritical fluid is also proposed. Particularly, an article for a surface modification performed in coexistence of alkanethiol in a supercritical hydrothermal synthesis (Technoarch's Patent) field is disclosed. When the coexistence of hexane thiol with an aqueous solution of Cu(NO3)2 laid in a supercritical state results in synthesis of Cu particles by reduction and stabilization thereof by hexane thiol in situ. In this case, the thiol acts as a reducing agent to coordinate the hexane thiol with the surface of the generated Cu nanoparticles. This is a well-known coordination with metal (Kirk J. Ziegler, R. Christopher Doty, Keith P. Johnston, and Brian A. Korgel, “Synthesis of Organic Monolayer-Stabilized Copper Nanocrystals in Supercritical Water”, J. Am. Chem. Soc., 2001, 123, 7797-7803). Surface modification of SiO2 with alcohol with synthesis thereof is also known (the same reference as above Kirk J. Ziegler, R. Christopher Doty, Keith P. Jonston, and Brian A. Korgel, “Synthesis of Organic Monolayer-Stabilized Copper Nanocrystals in Supercritical Water”, J. Am. Chem. Soc., 2001, 123, 7797-7803).
The present inventors have proposed a nanoparticle synthesis in supercritical water for synthesizing highly crystalline particles of nano-size by adapting supercritical water as a reaction field for hydrothermal synthesis, but not referred to a process for modification of the surface of the produced particles or synthesis of organically modified particles by a reaction with an organic matter. There are frequently reported that high-temperature, high-pressure water forms a homogenous phase also with an organic material and that water functions as an acid or basic catalyst to progress an organic synthetic reaction even in no catalyst in a high-temperature, high-pressure field. However, the process for reaction between an inorganic material and an organic material has not been reported yet.
With respect to fine particles, particularly, nanoparticles, the usability of which is expected because of various useful properties and functions, a number of synthetic methods have been proposed and developed, including the supercritical synthetic method. However, a method for recovering the thus-synthesized fine particles or nanoparticles, and a method for dispersing and stabilizing the fine particles as they are without coagulation after recovery are needed. At the time of use, they must be satisfactorily dispersed in a resin, plastic or solvent. Particularly, nanoparticles synthesized in water are not easily recovered from water since they frequently have hydrophilic surfaces. The nanoparticles or the like have the problem of unfamiliarity with organic solvents, resins or the like.
In order to satisfy these needs, it might be necessary to modify the surface of nanoparticles with organic materials according to the respective purposes. For example, modification with the same polymer as the resin, or donation of the same functional group as the solvent is desirable. If the nanoparticles can be surface-modified in water, the separate recovery of the nanoparticles from water is facilitated. However, although it is desirable for the surface modification of nanoparticles synthesized in water with an organic material that the organic material forms a homogeneous phase with water, the modifying agent usable therefor is limited to an amphipathic surface active agent, a lower alcohol soluble even to water, and the like. Further, even if the nanoparticles are recovered by any method, the recovered nanoparticles are extremely easy to coagulate, and it is difficult to redisperse the nanoparticles coagulated once even by use of a dispersant. The surface modification of such nanoparticles is entirely difficult.
It is well-known that water and an organic material form a homogenous phase in a high-temperature, high-pressure field and, for example, alcohol and sugar, carboxylic acid and alcohol, or carboxylic acid and amine non-catalytically cause a dehydration reaction in high-temperature, high-pressure water. However, it is not known that a reaction is caused between hydroxyl group on the particle surface and the organic material in this condition.
Thus, it is needed to develop the method for introducing a required desirable functional group to nanoparticles at the time of synthesis thereof in water. As a result of the earnest studies, the present inventors found that synthesis of metal oxide particles in a high-temperature, high-pressure hydrothermal synthetic field in the coexistence of an organic material results in surface-modified fine particles having the organic material strongly bonded with the particle surface by the occurrence of a homogenous phase reaction between the particle surface and the organic material. It is also found that the resulting nanoparticles can be phase-separated from water with the remaining organic material, and easily recovered after cooling because they are organically modified. The present invention has been accomplished based on such knowledge.